1. Field of the Invention
The present invention relates to a multilayer positive temperature coefficient thermistor device in which a plurality of semiconductor substrates having positive resistance-temperature coefficient are stacked with each other, and more particularly, it relates to improvements in materials of electrodes which are in ohmic contact with the semiconductor substrates and a stacked structure including the plurality of semiconductor substrates.
2. Description of the Background Art
For example, Japanese Patent Application Laying-Open No. 3-145920 (1991) discloses a multilayer positive temperature coefficient thermistor device which is of interest to the present invention. FIG. 3 shows such a multilayer positive temperature coefficient thermistor device 1.
Referring to FIG. 3, the thermistor device 1 comprises a plurality of semiconductor substrates 2. Each semiconductor substrate 2 is obtained by adding a slight amount of a rare earth element such as lanthanum, cerium, yttrium or samarium to a material which is prepared by partially replacing barium forming barium titanate with strontium for attaining a semiconductor state, and firing this material, for example. As shown in FIG. 4, each semiconductor substrate 2 is in the form of a rectangular plate, which is provided with ohmic electrodes 3 and 4 on surfaces thereof. The first ohmic electrode 3 is formed to extend from a first major surface toward a first end surface of the semiconductor substrate 2, while the second ohmic electrode 4 is formed to extend from a second major surface toward a second end surface of the semiconductor substrate 2, thereby providing L-shaped sections respectively. The ohmic electrodes 3 and 4 are mainly made of silver, and contain at least one of bismuth, antimony and zinc, which is added to provide an ohmic property.
In the thermistor device 1 shown in FIG. 3, six semiconductor substrates 2 are stacked with each other. In more concrete terms, directions of the semiconductor substrates 2 as stacked are so selected that those of the ohmic electrodes 3 and 4 extending toward end surfaces of the same sides are in contact with each other. Such a stacked state of the semiconductor substrates 2 is maintained by conductive holders 5 and 6. These conductive holders 5 and 6 are mounted on respective end portions of a stacked structure which is formed by the semiconductor substrates 2, to bring the respective end portions of the semiconductor substrates 2 into pressure contact with each other.
The conductive holders 5 and 6 also serve as external terminals of the thermistor device 1. The first conductive holder 5 comes into electrical contact with the first ohmic electrodes 3 which are formed on the respective semiconductor substrates 2 respectively, while the second conductive holder 6 comes into electrical contact with the second ohmic electrodes 4 which are also formed on the respective semiconductor substrates 2 respectively. Therefore, the six semiconductor substrates 2 are electrically connected in parallel with each other by the conductive holders 5 and 6.
In the aforementioned thermistor device 1, it is possible to change the combined resistance value provided by the overall thermistor device 1 by changing the number of the semiconductor substrates 2.
While each of the semiconductor substrates 2 forming the thermistor device 1 shown in FIG. 3 is provided with the ohmic electrodes 3 and 4 having L-shaped sections as shown in FIG. 4, such a semiconductor substrate 2 may be replaced by a semiconductor substrate 2a shown in FIG. 5. This semiconductor substrate 2a is provided with ohmic electrodes 3a and 4a which extend only on respective major surfaces thereof. According to such ohmic electrodes 3a and 4a, an operation for forming the same is simplified as compared with that for the ohmic electrodes 3 and 4 shown in FIG. 4, while it is possible to improve reliability in electrical connection since no disconnection is caused on edge portions.
However, the aforementioned multilayer positive temperature coefficient thermistor device which is disclosed in Japanese Patent Laying-Open No. 3-145920 (1991) has the following problems to be solved.
First, the silver forming the ohmic electrodes may migrate into the semiconductor substrates during employment of the thermistor device, to cause an electrical short across the first and second ohmic electrodes provided on each semiconductor substrate.
Further, the stacked state of the plurality of semiconductor substrates is maintained by the conductive holders. Thus, it is troublesome to handle the plurality of semiconductor substrates, which are not connected with each other before the conductive holders are mounted on the stacked structure.